Sn was added to a 1% Rh/Al 2 O 3 catalyst (D=75%) by controlled surface reaction (CSR) [1, 2] of Sn(n-C 4 H 9) 4 in amounts corresponding nominally to 1/4, 1/2, 1 and 2 monolayers. The Sn-Rh catalysts were characterized by hydrogen chemisorption, TEM, CO-FTIR, Mössbauer spectroscopy [3] and in test reaction of methylcyclopentane (MCP) [4]. Three different Sn phases were found after reduction (Table): SnO 2 , Sn-rich and Sn-poor RhSn alloy [5]. Tin transformed entirely to SnO 2 upon oxidation. The effect of oxidation (373 K) and reduction (at 473 K) appeared to be reversible. The 1 monolayer Sn-Rh sample behaved in MCP transformation similarly to a catalyst with small Rh ensembles while the product pattern of other Sn-Rh catalysts was similar to that produced by larger Rh ensembles (Table). The suggested model is as follows: Sn was deposited selectively on low-Miller-index microfacets of Rh when its introduced amount was lower than 1 monolayer. Increasing the introduced amount of Sn led to its deposition also on high-Millerindex sites (random deposition).